The soil in which you choose to grow
your Bonsai is in effect its sole life support and as such you cannot
pay too much attention to its preparation. It should be considered very
carefully in terms of the function it has to perform for a particular
species of tree at a particular age.

Before going into the specifics of soil preparation there are
certain key points to take into account. The soil has to physically as
well as nutritionally support the tree; it must be able to drain
freely; it needs to contain oxygen in the form of air; it should have
the capacity to remain comfortably damp without becoming waterlogged;
it should have within its properties a 'buffer' capable of reserving
nutrients in a dissolved state; it should be possible to control its pH
value; it should not be unsightly in its appearance; it should be
capable of retaining its physical state for as long as possible to
reduce any tendency to compact.

Having said this, there is no single bonsai 'magic mix,' and most
bonsai enthusiasts tend to arrive at their own conclusions concerning
the right soil. This, to a great extent, is dependent on the local
availability of the necessary ingredients. What is important is that
the function of the soil is fully understood and some of the points
mentioned will serve as a useful guide.

I use Grimson stone and crushed lava rock for my aggregate. I
have heard of people using cat litter, Turface, or Oil-Dri which are
all calcinated clay. My experience with calcinated clay here in Florida
is with our summer rains it stays wet and breaks down too quickly. Clay
is good at holding nutrients in the soil but it also holds the salts
and other unwanted ingredients from some of your fertilizer which could
toxify your soil, so I found a little clay is good but to use it as
your main aggregate I didn't care for. I found the natural rough
aggregate works best.

I wash my aggregate, dry it and then sift it through a 1/4"
screen, what stays becomes my aggregate for my coarse mix, what goes
through is sifted again through a 1/8" screen, what stays becomes my
medium mix and what goes through is for my fine mix.

For my organic matter I use dried, aged, chipped pine bark. I
sift it through a 1/4" screen, what stays goes into my coarse mix and
what goes through is sifted again through a 1/8" screen, what stays is
put into my medium mix, what goes through is my fine mix.

The proportions of each are as follows:
75% aggregate
20% pine bark
5% Turface

The proportions of each of the basic ingredients, however, will
vary according to the species of tree grown and as to which stage of
development the tree is in. Younger trees in early stages may need more
organic matter for faster growth, whereas your more mature trees will
do better in less organic matter to control growth. Conifers generally
prefer more aggregate for better drainage, maples and other deciduous
trees prefer more organic matter. Experimentation, followed by
observation, is the best way of finding out which soil mix is best
suited to a particular species and location. Although one can have good
results by using just a standard mix.

For the hobbyist with a few trees one would probably be better
off finding a source of standard, ready made soil, suitable to your
area and then amending it to your particular species and age of tree.
If you have a large collection then doing all the work to make your own
soil might be beneficial.